BACKGROUND OF THE INVENTION
The present invention generally relates to a drilling machine for making a taphole in a furnace and, more particularly, to a drilling bit for use in the furnace taphole drilling machine.
It is well known to those skilled in the art that a taphole at or near the bottom of a blast furnace is, during the operation of the furnace, during that is, the manufacture of pig iron, closed by a mud material which has been introduced into the taphole in the form of a refractory slurry and which is sintered and, therefore, hardened by the heat evolved in the furnace being operated. Each time molten iron is tapped through the taphole, the thus hardened mud must be removed by the use of, for example, a drilling machine. For this purpose, the drilling machine has heretofore made use of a drilling bit having a bit head provided with cutting tips made of steel.
With the drilling bit having the steel cutting tips, it has been found that, since the mud is hardened as described above, a lot of time is required in making the taphole in the blast furnace with substantial reduction in drilling efficiency.
The Japanese Utility Model Publication (Examined) No. 42811/1975, an unexamined version of which was first laid open to public inspection under No. 110408/1973 on Dec. 19, 1973, discloses a drilling bit having a bit head onto which are welded a plurality of tips made of super hard alloy, the resultant lines of welding being in turn deposited with metal from a welding rod to form a padding. The super hard alloy in the present invention is also called a cemented carbide or a high hardness alloy. As compared with the drilling bit of the first described type, the drilling bit having the construction according to the Japanese Utility Model Publication is satisfactory in its performance. However, some disadvantages have still been found. One of them is that, since the tips are rapidly heated and cooled during the drilling or tapping operation to form the taphole in the blast furnace, these tips are susceptible to cracking. Another disadvantage is that the formation of the padding requires a long time during the manufacture of the ultimate drilling bit. Furthermore, the recent practice to improve the economy and material saving is to reuse the once-used drilling bit after a padding of stellite has been formed on the worn cutting tips. The reuse of the worn drilling bit, however, is difficult partly because of insufficient bondability between the stellite and the super hard alloy and partly because of the formation of an oxidized film on the surface of the padding.
In view of the foregoing, the drilling bit largely employed nowadays in making the taphole in the blast furnace has a construction manufactured by depositing stellite on the bit head to form a stellite padding and then grinding the stellite padding to provide a sharp cutting edge. With the drilling bit, once the cutting edge wears, the bit can be reused by re-depositing stellite on the worn cutting edge and then grinding the re-deposited stellite padding to form the cutting edge. However, this technique has been found to require a lot of time, and a relatively large amount of material for making the padding and in the preparation resulting in an increased processing cost.
Moreover, the yield of the material used for the padding is relatively so low which results in a increased cost of the drilling operation.
Alternatively, it is possible to use a drilling bit having tips made of either stellite or a super hard alloy composed of tungsten carbide and cobalt, which tips are soldered to the bit heat. However, where the solder for connecting the tips to the bit head is employed is a silver or copper solder which is generally used in making a drilling bit for use in a rock drilling machine, the solder tends to be melted, when such a drilling bit is used in drilling the taphole in a blast furnace, with the consequent separation of some or all of the tips from the bit head, since the drilling bit is heated to a temperature more than 1,100.degree. C. during the tapping operation. In view of this, such a drilling bit cannot practically be used in the preparation of the taphole in the blast furnace.